Such units, as is well known, typically comprise two coaxial confronting discs defining a toroidal race and a plurality of rollers each engaging the discs to provide a variable transmission ratio between the discs in accordance with the attitude of the rollers with respect to the common axis of the discs. The discs are urged towards each other by a suitable end load in order to provide sufficient traction between the toroidal race of each disc and the rollers. By varying, usually in unison, the common attitude of the rollers with respect to the main, common axis of the input and output discs the transmission ratio may be changed. It is in practice necessary to change the attitude of the rollers by steering them in helical paths with respect to the discs. In a common form of construction, each roller is mounted in a carriage which has a rotary axis lying in a plane substantially normal to the main common axis of the discs. Each carriage may have its rotary axis defined by, at one end of the carriage, a sliding pivot such as a ball guide, and at the other end a spherical ball joint comprising a race and a ball which may be connected to a rocker arm which is or can be pivoted in unison with corresponding rocker arms for the other carriages in order to achieve a change in the attitude of the rollers and accordingly a change in the transmission ratio between the input and output discs.
Toroidal race rolling traction units of the general type are described in GB 979062 and GB 1069874; an arrangement including rocker arms is described in GB 2107009.
In many designs of this general kind there is difficulty in providing sufficient working clearances around the ball socket of the carriage for the roller in order to provide full angular travel of the carriage during a ratio change and yet to keep sufficient material to support the ball on the rocker against severe impulse loading. It is also necessary though difficult to maintain proper clearances between the carriage and the rocker at extreme limits of the range of variation of the attitude of the rollers and accordingly the transmission ratios that they provide.